The invention relates to food serving systems, and particularly to food serving trays of the type used in such systems in transportation vehicles such as aircraft and trains and in institutional feeding locations such as hospitals. More particularly, the invention relates to food serving trays of the type which are adapted to permit the heating of one or more food items thereon while other food items are either chilled or isolated from the heating element. Examples of such trays include those disclosed in Schulz et al U.S. Pat. No. Re. 30,623 and Dodd et al U.S. Pat. No. 4,346,756 wherein a heater shelf is moved above the tray surface and under a dish which is lifted by the heater shelf. Tricault French Pat. No. 1,219,352, Colato et al U.S. Pat. No. 4,005,745 and Naimoli U.S. Pat. No. 3,608,770 teach the concept of forming an aperture in a tray to restrain the dish while allowing it to be lifted by a heater shelf positioned under the tray. Although the aforementioned techniques of heating a dish are quite satisfactory, there are some potential users who would prefer a system which does not have a hole in the tray or a discontinuity in the outer rim of the tray through which liquid spilled on the tray could conceivably get on a passenger. For example, in some trays which have a hole through the tray bottom surface to support a dish, the tray bottom surface is not substantially continuous. It is thus possible, where such a tray has only a relatively narrow support portion at one end of the tray, that accidental movement of such narrow support portion beyond the edge of the tray table on which the tray is resting will cause the tray edge to drop quickly and cause a cup or glass of liquid on the tray to spill. Other potential users would prefer a system which eliminates all chance that a dish will not be properly positioned on the tray when the trays are being heated. It is quite unlikely that a dish will be bumped out of its proper position as a cart containing the trays is transported, for example, by a truck to an aircraft. However, it is possible, although it rarely happens, for a food service worker to carelessly fail to place a dish in its proper cooperative orientation relative to the tray, thus possibly preventing the dish from contacting the heater shelf and being properly heated. Other potential users are concerned about the expense of the specially shaped dishes required for the aforementioned systems. They are relatively expensive since they must have flat bottoms for good contact with the heater pads and must remain stable over numerous cooking cycles. Also, some difficulty exists when the in flight serving person picks up trays. In some cases the passenger has moved the hot dish out of its required position on the tray, thus causing interference with the heater shelves during reloading of the tray into the cart. Furthermore, most designs require reloading in a specific orientation, thus introducing an additional burden on the serving person as the trays are being returned to the carts after use.
An alternative to heating the food in a dish which must be lifted relative to a tray by a heater pad is to heat it in a formed recess in the tray, with the heat passing upwardly through the tray, as taught by U.S. Pat. No. 3,886,346 and by Tricault French Pat. No. 1,205,953. Although relatively low cost disposable trays are known and disclosed in U.S. Pat. No. 3,647,104, for example, it is preferable, from a performance and long term cost standpoint, that a heavier weight, temperature stable, reuseable tray be used. To facilitate cleaning of the tray and to avoid scratches and other damage to such a reuseable tray by a user's eating utensils, it is highly desirable to use a disposable insert dish in a tray recess, as disclosed by Tricault French Pat. No. 1,234,794 and U.S. Pat. Nos. 3,305,124, 3,877,603 and Re. 30,962.
It has been noted that food on a tray can be heated by applying heat under the tray. However, it would seem obvious that the bottom of such a tray could be quite hot to the touch if handled shortly after the heat source was disconnected. Such a situation could be expected in an aircraft, especially on a short flight where serving time is limited. Also, since most aircraft tray tables are made of low melting point plastics, such as ABS, a hot tray could damage a tray table on which it is placed. For example, it is desirable for microbiological purposes and palatability, to heat most foods to about 165.degree.-185.degree. F. (74.degree.-185.degree. C.). However, to achieve such a food temperature in a reasonable time in a particular prototype unit seems to require that the tray to be heated by a 300.degree. F. (149.degree. C.) surface temperature heater element. Obviously, however, the specific temperature achieved at the heater surface is dependent, for example, on heater surface area, watt density, the materials being heated and the total heating time. Such heating could cause the bottom surface of the tray to reach a temperature of about 200.degree.-210.degree. F. (93.degree.-99.degree. C.). Naturally, manual contact with the bottom of a tray while at such a high temperature should be avoided. If a heat conductive, fast cooling material is used for the tray, the tray bottom would cool relatively quickly but it would also quickly extract heat from the food, rendering it less palatable. If a material with low heat conductivity is used, the heating time required would be increased considerably.